I've read the arcs, but I'm really not feeling this plan of action from Marvel - if the rumors are true. Yeah, I would love to see the movie happen. But this would be the first solo Hulk film since the Avengers. And it should be planned with Ruffalo in mind and not be a total CGI/motion capture extravaganza. There should be focus on both Bruce Banner and the Hulk, and not just one or the other. Only focusing on the Hulk is simply too much to risk for this first movie.
The Avengers has made millions of new fans and many of them haven't read the Planet Hulk/World War Hulk arcs. They'd be disappointed to hear that there’s going to be a huge focus on the Hulk. Sure, he can have tons of characterization but for many people, including myself, the lack (or entire absence) of Banner will be a deal-breaker.
And I really want to see more of Ruffalo and his characterization of Bruce Banner. We’ve barely seen anything of him, and if the next solo Hulk film involves very little of Banner, I'm not really for that. Ruffalo's Banner brings a meek but dangerous charisma to the character, and a Jekyll/Hyde dynamic, that would be almost (if not entirely) absent in a “Planet Hulk” film.
Not only this, but in the comics, Banner and the Hulk are two different personas. But as far as the MCU goes, grounded in reality as it stands, the Hulk is simply an experiment gone wrong. The Hulk IS Banner, but he just can’t process his thoughts and emotions. There’s no split-personality going on there. In the comics, Banner developed something of an MPD while he was very young and part of this manifested as the Hulk. This hasn’t been touched on at all in MCU canon, and with only the “Avengers 2” between Ruffalo’s first appearance and a possible “Planet Hulk” film, it may be difficult to bring in a whole new Hulk persona and create some kind of backstory to go along with it. No hint of a backstory has even been established yet and if this is to happen during Avengers 2 or such, it might not be rich enough. There's a ton of other characters the studio has to juggle with in the same film. Stuff like this really belongs in its own movie. So how they’d create the split-personality thing without it being half-baked is beyond me - unless, of course, Banner has full control of the Hulk and such. That's the only way I see this working, but ymmv. The studio has their own idea.
I really think, if anything, Banner's backstory should be explored in the next film before doing something like a Planet Hulk storyline. Even then, the whole concept of Banner and the Hulk being separate entities seems a little odd for an MCU film. It works fine in the comics, but I don't see it working in the cinemas.
/end of thought spillage (I geeked out more than I thought I would

)